Enhanced output light source



BA E@ Jan. 6, 1970 J. c. HOLME ENHANCED OUTPUT LIGHT SOURCE Filed Nov. 3, 1967 United States Patent O 3,488,130 ENHANCED OUTPUT LIGHT SOURCE John C. Holme, Wayne, NJ., assignor to Astrosystems International, Inc. Filed Nov. 3, 1967, Ser. No. 680,397 Int. Cl. F23j 7/00; F23q 2/32; F23r 1/00 U.S. Cl. 431-4 13 Claims ABSTRACT OF THE DISCLOSURE Intense visible light output is provided by a transparent combustion chamber through the heating to incandescence of visible light emissive materials within the chamber. The total transverse cross-sectional area of the exit orifice of the chamber for the burning gases at its narrowest region is considerably less than that of the chamber at its narroWest region.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a light source for producing intense illumination over a large area. More particularly, the invention has application to a light source capable of illuminating large land areas under airborne vehicles and adapted for simple, efficient mounting in and operation from the aircraft. The illumination of interest is that portion of the electromagnetic wave spectrum visible to the human eye, i.e., luminous flux, a quantity measured in lumens.

DESCRIPTION OF THE PRIOR ART With the exception of lasers, all light sources produce significant amounts of invisible radiation. Lasers, on the other hand produce highly focused beams not readily adapted to producing the large land area illumination from aircraft of interest herein. Visible light sources that can illuminate large areas -should provide as large a ratio of visible radiation to total radiation produced as possible under the circumstances-and such a ratio is a figure of merit.

Two basic mechanisms exist for producing light. The first is by heating a solid material to an incandescent state, i.e., one in which it will glow with a white spectrum. Such processesv are heavily temperature dependent. The second is by applying sufiicient energy to an atom or a molecule to cause a transition of its electron or electrons to an energy state higher than its'normal, or rest statethis process is not necessarily temperature dependent.

The incandescent mechanism is termed continuous emission because the resulting radiation covers a wide spectral region without interruption, the length and intensity variation of which is a function of the source temperature. The common light bulb is one example of a continuous emitter. The electron excitation mechanism is termed discrete emission because it is confined to a relatively narrow portion of the spectrum. It can be triggered by other electromagnetic radiation (ultra violet or X-ray) or by electrical energy as in an arc discharge, e.g., the electrically energized, discretely emitting, xenon arc lamp. The conventional pyrothechnic flare provides light by a combination of these two mechanisms, i.e., discrete emission from thermally activated combustion gas constituents and continuous emission from hot solid particles generated by the combustion process or carried in the gas train or plume.

A light source for use as an aircraft-borne device for large area illumination should advantageously have several basic characteristics. If light is produced in whole or in part by incandescence, the source should be capable of operation at very high temperatures since the higher the ice temperature the greater the ratio of visible radiation to total radiation produced; moreover this ratio is a monotonic7 non-linear, increasing function of temperature (although the ratio in percent cannot exceed 30% no matter how high the temperature because of the narrow range of the visible region compared to the infrared) for grey body emission. The source should be self-contained and of reasonably small size for efficient airborne operation and handling. It should be capable of being focused so that application of the illumination to the area of interest may be achieved. It should have a high operating efficiency (a parameter not independent of the preceding factors). Most importantly, it must be capable of safe operation.

The electrical-filament continuous-emitting light bulb, the gas discharge arc lamp and the pyrotechnic flare are prior art that meet some of these requirements in varying degrees, but none achieve all of them.

An analysis of the pyrotechnic flare capability shows that it operates at an average combustion temperature approaching 4000 F. which is somewhat low for light applications; in addition some temperature is lost as the combustion products expand and cool. This light loss is offset somewhat by the resulting large plume surface; but this in turn leads to another disadvantageous featurethe inability to collect and focus any significant fraction of that emission upon the area of interest. In addition the open ame of the flare source makes aircraft carry unsafe during flare operation therebylimiting its tactical use to situations in which parachute suspended light point sources can be effective.

The electric powered xenon arc lamp and the metallic filament bulb provide relatively intense sources of focusable light. In almost all respects the assets and liabilities of these sources run counter to those of the pyrotechnic flare. The electric lamp is a small source, it is focusable, it is not droppable, and its duration is relatively long cornpared to the flare. The major disadvantage of the lamp system is its weight and size--the major assist is its intensity and focusability. The electrical light system is thus limited to situations in which a large aircraft can be deployed to carry a rather ineicient system which is not, basically, self-contained.

In order to overcome the drawbacks selectively applicable to the various conventional prior art sources it was necessary to provide a new source which is as hot as a filament source but larger than an electric source, more self sufficient in operation than the electrical system, safer to carry than an open flame system but as eicient in operation as that system. The provision of such a new light source is the object of this invention.

SUMMARY OF THE INVENTION The light source comprises an optically transparent chamber or envelope or material capable of withstanding intense heat without starting to boil during the requisite operating period of the light source. Fused quartz is an example of such a material. One or more exhaust orifices or nozzles is provided at the exhaust end of the chamber for combustion gases to escape. At the other end a fuel and oxidizer capable of producing intense heat, e.g., above 4000 F., is introduced and ignited. As a consequence the combustion products may be heated to incandescence as may the inside wall of the chamber. Moreover materials are introduced into the transparent combustion chamber to enhance spectral emissibility in the visible region. Such materials preferably are discrete emitters introduced into the chamber as salts suspended or dissolved in the fuel or otherwise independently introduced therein.

The exhaust orifices or nozzles are seen as constricted regions by the burning gases passing out of the combustion chamber. As a consequence, substantial back pressure is built up in the chamber. This results in three significant advantages in light source operation. The increased back pressure results in a larger concentration of continuous and discrete emitting molecules per unit volume within the transparent chamber than would be present otherwise. Moreover, when the intensity due to such concentration can no longer be increased by this means, the pressure in the chamber broadens the spectral line of whatever discrete emitters are present to thereby further enhance visible light output. In addition, the boiling point of the material of the inside wall of the chamber is greater with increased pressure. Thus the tube may be operated with less loss of tube material and less chance of structural failure.

BRIEF DESCRIPTION `OF THE DRAWINGS For a better understanding of the present invention, together with other and further objects and advantages thereof, reference is made to the following description taken in connection with the accompany drawings. In the drawings:

FIG. 1 is a side view of the lamp housing mounted below fuel and oxidizing tanks;

FIG. 2 is a cross sectional lview of the arrangement shown in FIG. l and is taken along liney 2--2 of that figure;

FIG. 3 is an alternative form of the invention showing a tank container and a control unit mounted exterior to the lamp which is connected to the control unit by a long supply cable.

FIG. 4 is a cross sectional view showing the details of the `combustion chamber, the mixing chamber, and 'the supporting fixtures; and

FIG. 5 represents in its curves empirical data relating brightness and pressure in a light source such as that of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, FIGS. 1 and 2 show a large tank which'lmay hold an oxidizer such as gaseous or liquid oxygen. Mounted below tank 10 is a second tank 11 which holds a fuel, either gas or liquid. A third tank 12 may be mounted on the other side of the -assembly and may hold other gases or liuids such as nitrogen for pressurizing the liquid fuel. The light source itself 13 is -mounted directly below tank 10 and may be positioned between two reflectors y14 and 15. The reflectors help to intensify the light directly downwardly and they form a barrier which prevents the light source 13 from illuminating the airborne vehicle which is supporting the source. A lens 16 may be mounted below the reectors, but -this is not always necessary and may be omitted. Two or more supports 17 and 18 may be secured to tank 10 for supporting the arrangement on the ground prior to being airborne. One or more brackets 20 and 21 are secured to the tank 10 for support cables 22 when the device is lowered from an airplane or helicopter during flight.

FIG. 3 shows the preferred -arrangement whereby a light source 23 is provided with the three tanks 10, 11 Iand 12 housed within a compartment 24 having a control panel for operation by an operator within the aircraft. The housing 24 is supported in the aircnaft and is connected to the light source 23 by means of a cable 26, this cable including flexible conduits for transmission of the fluids and an electric transmission line for providing the spark plug with electrical energy for igniting the fluid mixture.

The introduction of discrete emitters directly into the burning gases may be accomplished in several ways. Liquid oxygen is a preferred oxidizer land a hydrocarbon fuel such as the jet engine fuel .IP-4 is a preferred fuel (and readily available where jet aircarft operate). Many metallic salts providing discrete visible emission are soluble in .TP-4. With such salts already dissolved in the fuel in its tank, the descrete emitters are automatically introduced into the combustion chamber and thus incorporated into the stream of burning gases. The discretely emitting additive used should be one that produces copious amounts of radiation in the visible portion of the spectrum. Particularly appropriate, therefore, is sodium which is a persistent line emitter; boron compounds possess many lines distributed throughout the visible spectrum. Other discrete emitters -may be used with those already mentioned to modify the distribution of radiation in the spectrum by introducing additional lines to` thereby enhance the quality of the radiation. Some of those discrete emitters not soluble in the particular fuel used may nonetheless be employed as a colloidal dispersion.

Referring now to FIG. 4, the detailed construction of the lamp is shown. The inner quartz cylinder 28 is shown. Surrounding this tube 28 is an outer tube 30, also of fused quartz, which is designed to be a permanent transparent retaining tube. The inner tube 28, because of the high temperatures applied to it may sublime or otherwise disintegrate and for this reason may require renewal after a period of service.

The exit end of the quartz tubes is surrounded by a block of metal 31. An added liner 32 of tantalum-tungsten alloy is mounted adjoining tube 28 and contains orifices 33 of cross sectional area in their narrowest regions that is considerably less than that of tube 28. Orifices 33 permit the burned compounds to escape into the atmosphere. At the other end of the tube 28 a block 41 includes a mixing chamber 34 where the fuel and the oxidzer fiuids are mixed rand then discharged into the interior of tube 28. The mixing chamber 34 is connected by a first conduit 35 to a tank containing fuel fluid. A solenoid valve 36 is mounted in series with conduit 35 so as to open or close the supply line to the mixing chamber. In a similar manner, conduit 37 supplies the oxidizing liuid to the mixing chamber in series with a solenoid operated valve 38.

The input terminal `and the output termina-l are mounted on either ends of the combustion tube 28 and may be clamped together by any suitable means such as bolts 40. The mixing chamber 4and the valves 36, 38 are all mounted in a recess in terminal block 41. An expendable washer 44 of heat insulation is placed between the ends of quartz cylinder and block 41. Clearance may be allowed between the outside diameter of tube 28 and the internal diameter of outer -tube 30 to accommodate thermal expansion. Similarly, axial clearance may be provided along the direction of the longitudinal axis of tube 28 between its ends and their adjacent flanges.

Appropriate seals, not shown, are provided, in manner well known to those of skill in the art, to prevent the escape of combustion gases. These may be located, for example, in, on, or between flanges 31 and 41 and their respective contact zones on tube 30. l

The operation of this type of light source is as follows: after being lowered from the aircraft, the valves 36 and 38 are opened by remote control and a mixture of inflammable fluids and solution or dispersion of discrete emitting molecules is delivered to the combustion chamber. At this point current is applied to the high tension coil 42 and the spark electrode 43 and a spark jumps across the gap to ignite the combustible mixture in chamber 34. This flame raises the temperature inside tube 28 and causes incandescence in any continuously emitting particles (such as carbon from the hydrocarbon fuel) in the burning gases and excites the discrete emitters carried in the burning gases to emit visible radiation. The burned gases pass from their point of ignition through tube 28 and are ejected through exit orifices 33. This action provides a brilliant light which may be applied to any large land area.

The enhanced output intensity of the light source may be, in accordance with the principles of the invention, un-

derstood from the following discussion and the data represented by the curves for FIG. 5. When the concentration of discretely emitting atoms in the burning gases is sufficient to cause the intensity of the emitted spectral line to equal that of a blackbody at the radiated wavelength, any further increase in concentration cannot increase the intensity at that wavelength. Rather, an increase in intensity is, and can only be produced by exciting radiation at other wavelengths. Pressure broadening, which results from the small shift in atomic energy levels due to collisions, can produce such a result; pressure broadening is manifested as continuous bands of radiation located close, to and on either side of, the main spectral line. Thus lin the operation of the light source of the invention, increasing concentration produces an increase in emission intensity in the side bands, even after the main line can no longer increase in intensity. Thus, pressure broadening is an effective mechanism for increasing the luminous output of a hot gas when the emitted line lies in the visible region of the spectrum. Pressure broadening may be induced by raising the total gas pressure which, in FIG. 4 is achieved by constricting the flow ex-it orifices 33. The effectiveness of this technique is illustrated in FIG. 5 which shows empirical data obtained in actual reduction to practice relating brightness to total gas pressure. These curves show that the luminous output increases by more than a factor of two when the pressure is increased from 26 to 36 pounds per square inch (curve 60 represents the brightness v. pressure characteristic at the upstream end of the combustion chamber and curve 70 that at the downstream end).

The introduction of the discrete emitter additive directly in the fuel is particularly convenient since no special structure is required. However, since many types of discrete emitters may be used, whether as solids, liquid, vapor or colloidal dispersions, other means for introduction into the combustion chamber may be appropriate for particular ones. For example, an emitter in liquid form such as boron trichloride, or which can be readily dissolved in large quantities in a carrier uid, e.g., aqueous sodium hydroxide, can be directly injected into the combustion chamber through a separate inlet tube. Flow control is provided by a porous ceramic element in the inlet tube so that a reasonable pressure drop can be maintained at the low ow rates that would be involved. The discrete emitter may also be introduced as liquid droplets or solid :particles of a colloidal suspension with a gas carrier (such as nitrogen under pressure). Inlet tubes may be coupled to upstream end of tube 28 in an arrangement to provide a vortex injection of the emitter laden gas. In such an arrangement the added advantage is obtained of the cooling of the inside surface of the quartz tube 28 by the carrier gas to keep, thereby, the ablation of the quartz to a minimum. Should the emitter additive be soluble in the fuel, it may be readily introduced during the operation of the tube rather than beforehand. Thus, the additive may be introduced directly into the fuel line (through a porous ceramic flow control element) just after the solenoid valve that controls fuel input to the mixing chamber, Such an arrangement has the advantage of keeping the fuel tank free of contamination. Another way in which the discrete emitter may be introduced into the fiow of burning gases uses the lpintle features of the inventions of Stanley Lehrer disclosed in his patent application Ser. No. 655,025, filed July 21, 1967, entitled Incandescent Light Source and his patent application Ser. No. 686,365, filed of even date herewith, entitled Steady Intensity Incandescent Light Source, both of common assignee herewith. In `both applications a graphite pintle is disposed within the combustion chamber to enhance or maintain light output.

A discrete emitter additive may be used to impregnater the surface of the pintle. Under the intense heat in the combustion chamber the additive evaporates from the pintle surface and fills the chamber with vapor. Many types of additive may be added to a graphite matrix and their relative concentrations can be readily controlled.

It will ybe recognized that the pressure within chamber 28, and thus the brightness of the tube output in the sense of the curves of FIG. 5, is a function of the amount of output constriction provided by the output flow orifices 33. The shape of the orifice may be varied as structural convenience may suggest; if desired a single orifice .may be used. The significant factor is that it provide a suliicient constriction to the flow of burning gas to introduce the requisite back-pressure in the chamber to produce the requisite brightness with the discrete emitter used. In order to vary the effective cross-sectional area of the output orifices 33, should such variability be valuable in a particular light source, remote controlled mechanical plug valves may be used to replace the orifices.

The foregoing disclosure and drawings are merely illustrative of the principles of this invention and are prayed not to Ibe interpreted in -a limiting sense.

What I claim as my invention is:

1. An enhanced output light source, comprising: a combustion chamber of refractory material transparent to visible radiation having an input region and an output region; means for introducing `burning gases into said chamber at said input region to flow to said output region; means for introducing discrete emitter material into said burning gas ow; and means at said output region for generating back pressure in said chamber.

2. A light source as recited in claim 1 wherein said `back pressure generating means produces sufficient pressure within said chamber to broaden a spectral line of said discrete emitter material.

3. A light source as recited in claim 1 wherein said back pressure generating means comprises a physical constriction at said output region restricting the ow of said discrete emitter material containing burning gases.

4. A light source as recited in claim 1 wherein said combustion chamber is a hollow cylinder of fused quartz and said input and output regions are the opposite ends of said cylinder.

5. A light source as recited in claim 4 wherein said back pressure generating means comprises a partial closure at the output end of said cylinder.

`6. A light vsource as recited in claim 5 wherein the open cross-sectional area of said partial closure transverse to the direction of said burning gas is smaller in magnitude than the smallest transverse cross-sectional area of said cylinder.

7. A light source as recited in claim 6 wherein said partial closure has a plurality of output orifices.

8. A light source as recited in claim `6 wherein said open area of said partial closure is sufficiently small to produce a back pressure sufficiently great in said hollow cylinder to broaden a spectral line of said discrete emitter material.

9. A process for enhancing the light output of an incandescent light source having an optically transparent combustion chamber within which an ignited mixture of fluid fuel and oxidizer ows as burning gases through said chamber, comprising the steps of introducing into said burning gas flow material that is a discrete emitter in the band of the spectrum to which said chamber is optically transparent, and producing back pressure in said chamber sufficiently great to broaden a spectral line of said discrete emitter material.

10. A process as recited in claim 9 wherein said step for introducing a discrete emitter material into said burning gas flow includes the step of dissolving said discrete emitter material in said fluid fuel outside vsaid chamber.

11. A process as recited in claim 9 wherein said step for introducing a discrete emitter material into said -burning gas flow includes the step of forming a colloidal suspension of said discrete emitter material in `said fluid fuel outside said chamber.

12. A process as recited in claim 9 wherein said step for introducing a discrete emitter material into said burning gas ow includes the step of introducing said discrete emitter material into `said combustion chamber independently of said fuel and oxidizer fluids.

13. A process as recited in claim 12 wherein said discrete emitter material is injected directly into said chamber as a vapor.

References Cited UNITED STATES PATENTS 427,187 5/1890 Ransperger 431-126 808,513 12/1905 COX 431-126X 3,075,577 1/1963 Cazalas 431--4 3,141,741 7/1964 Hoei et al.- 431-4X 3,393,967 7/1968 Fleishman et al 431-158 FREDERICK L. MATTESON, JR., Primary Examiner ROBERT A. DUA, Assistant Examiner 

